def isPrime(a):
    if a == 1: return False
    test = True
    k = a - 1
    while k > 1:
        if not a % k:
            test = False
            break
        k -= 1
    return test


n = input()
r = []
for i in range(len(n) + 1):
    for k in range(i, len(n) + 1):
        if i == k:
            continue
        a = int(n[i:k])
        if isPrime(a):
            r += [a]
print(max(r)if r else 0)

